The present invention relates generally to pulse radar measurement of liquid levels within process and storage tanks, and more specifically relates to an apparatus and method to permit level measurement within such tanks while providing secondary access for manual sounding that does not require removal of a radar instrument.
In many environments, various materials are stored and/or processed in tanks prior to or during their disposition. These materials include foods, beverages, pharmaceuticals and fuels. One particular and commonly-known use of such tanks involves the storage of fuel for various modes of transportation. These fuel tanks can range from a gas tank on a car to voluminous fuel tanks such as those located on Naval ships. In particular aircraft carriers house multiple fuel tanks. These are used to store jet fuel for the aircraft carried on the ship.
Regarding these fuel tanks on Naval ships, the ability to reliably determine the amount of the contents stored within a tank at any given time can be critical. Military readiness is often dependent on an adequate fuel supply. Additionally, accurate measurements of fuel usage directly translate into cost effectiveness in procuring a fuel supply for ships. As such, there are several benefits to accurately and reliably gauging the levels of fuel in these tanks. Thus, the Navy employs redundant measurement methods to determine and cross-check the fuel levels.
Methods for the determination of liquid levels may include visual examination or the use of various apparatus that gauge the level of the fuel. Current electronic measurement methods include differential pressure and magnetic float transmitter technologies, both of which rely on sensor contact with the process medium. However, several factors, both in the structure of the tanks and in the methods of measurement used, increase the difficulty in obtaining an accurate and reliable level reading. In the particular situation of jet fuel tanks of Navy ships, any visual reading is obstructed by the location of the tanks within the bowels of the ship and by the voluminous size of the tanks. Additionally, visual inspection of content levels lacks accuracy and can be time consuming. In order to avoid the problems attendant visual examination, various apparatus may be used to measure content levels in fuel storage tanks.
Differential pressure transmitters measure static pressure head of the liquid in the tank and pressure (if any) of gasses above the liquid. The arithmetic difference between these values is used to determine the liquid level. This is an inherently unreliable and inaccurate method because it depends on sensor location, ship motion and is an xe2x80x9cinferredxe2x80x9d level based on two other measurement.
Magnetic floats are mechanical in nature and involve a ball or buoy floating on the surface of the liquid, typically in a guide or sleeve mounted to the tank wall. These systems require cumbersome incremental metered filling of the tank to establish electrical resistance values that correspond to tank level. They are also prone to expensive clogging failures which require expensive invasive repair work.
However, certain additional problems, some of which are particular to jet fuel tanks on Naval ships, arise from the current manual back-up method and apparatus used for measurement of these liquid levels. Fuel measurement on Naval ships is performed manually through a sounding tube with a plumb line including an attached plumb bob. The sounding tube is commonly a one-and-one-half inch diameter pipe located as part of the infrastructure of the ship. The sounding tube extends within the fuel tank and is open to its contents. Thus, the liquid level within the sounding tube equals the liquid level within the fuel tank. This sounding tube is not necessarily a linear pipe, but may include bends in order to avoid various other infrastructure of the ship.
To measure fuel levels, the top of the sounding tube is opened. The plumb bob is then dropped through the interior of the sounding tube to the bottom of the sounding tube. Once the plumb bob is retracted from the bottom of the sounding tube using the plumb line, the liquid level may be read from the moisture level created by the fuel on the plumb line.
Certain problems arise from the plumb line method of liquid level measurement. First, jet fuel is clear and evaporates very rapidly, thereby enhancing the difficulty of accurately reading the plumb line to determine the associated moisture level. Second, the plumb bob may break off the plumb line during use. Due to the difficulty involved, as a practical matter detached plumb bobs are not retrieved from sounding tubes. As a result, subsequent plumb bobs used for measurement may be impeded from falling to the bottom of the sounding tube, resulting in reduced measurement range.
The present invention improves aforementioned primary contact methods of level measurement by using a non-contact level sensing gauge apparatus on a sounding tube to measure liquid levels in fuel tanks. There are several types of level sensing gauges. Examples include those that use radar transmitters, or ultrasonic waves. A high degree of accuracy has been achieved by the use of level-sensing gauges which monitor content levels by transmitting microwave pulses from an antennae toward the surface of the tank contents. These pulses are reflected from the contents and back to the antennae. Other radar gauges use a continuous wave rather than pulses. Radar signals are unaffected by noise, by extreme air turbulence, or by fluctuations in dielectric constant above a nominal minimum value, density, or conductivity. Even liquids having highly agitated surfaces or gas bubbles are usually reliably measured. Gas layering such as that produced by solvents or gases has virtually no adverse effect. Radar sensors are suitable for liquids, solids, powders, granules, dust, corrosive steam and vapors, regardless of the media characteristics, environment, low and high pressures or temperatures. As such, they are adequate for use in the sensing of the fuel level in tanks of Naval ships.
To this end, the present inventors devised the use of a non-contact radar level sensing gauge on the sounding tubes of Naval ships. However, problems again arise when attempting the back-up manual sounding method of level measurement, as is the practice in the Navy. For instance, removal of the level sensing gauge from the sounding tube in order to use the plumb line is unwieldy and time consuming.
Potential solutions include the use of a xe2x80x9cYxe2x80x9d-shaped fitting attached to the end of the sounding tubes. In this configuration, a level-sensing gauge is attached to one branch of the xe2x80x9cYxe2x80x9d to measure liquid levels by radar pulse while a plumb line could be simultaneously inserted through the second branch. However, the xe2x80x9cYxe2x80x9d-shape of the fitting may disrupt the pulse radar of the level-sensing gauge, resulting in inaccurate readings.
The present invention permits level measurement of fuel tanks on Naval ships while providing simple secondary access for manual sounding that does not require removal of the radar instrument. Additionally, the present invention is not susceptible to the above-discussed sources of inaccuracy. Further, the present invention allows multiple methods of liquid level measurement to be used, each without affecting the accuracy of the other, and while reducing or eliminating the other aforementioned reliability and service drawbacks of contact methods.
This invention thus solves the problems associated with known apparatus and methods for measuring levels of contents within storage tanks. The apparatus of this invention also satisfies the aforementioned needs that exist in the art as developed in the background of the invention.
The level sensing apparatus of the present invention includes two main components: (1) the radar level sensor itself and (2) a sounding tube adapter. The level sensor includes a transmitter disposed within a housing and an antenna, which is operatively connected to the transmitter. The transmitter emits the electrical and/or mechanical waves through the antenna. These waves are subsequently reflected from a surface and received by the antenna to measure the level of a certain substance, such as fuel, in a given container. In a radar level sensing gauge, the microwave pulses of radar travel down a waveguide and are subsequently reflected off the contents of the tank and return to the antennae via the waveguide. In the present invention, the sounding tube is used as a waveguide. The sounding tube adapter is operatively connected to the base of the housing of the level sensor. This sounding tube is analogous to a waveguide in the present application.
The level sensor is operatively connected to the sounding tube adapter by a tri-clamp which encompasses and tightens against a first flange that is integral with and circumferential about the housing, and a second flange that is integral with and circumferential about the sounding tube adapter. The level sensing apparatus, which includes the level sensor and sounding tube adapter, may then be mounted to a sounding tube through the use of a union fitting. The union fitting is threaded, including female threads which are compatible with the male threads of the sounding tube. A gasket in the union fitting forms a seal between the top of the sounding tube and the apparatus. Should the housing of the level sensor need to be rotated during use in order to open the top or to more easily view the LCD readout, the housing may be rotated without removing the apparatus from the sounding tube by loosening the tri-clamp.
In attaching the level sensing apparatus of the present invention to a sounding tube, the union fitting is first attached to the sounding tube as described above. The sounding tube adapter with attached level sensor is then screwed onto the union fitting. When properly attached, the antenna of the level sensor extends below the base of the housing of the level sensor, and is thus disposed within the sounding tube adapter. The configuration of a level sensing apparatus on a sounding tube allows for very accurate non-contact measurement of liquid levels in fuel tanks.
In another aspect of the present invention, the sounding tube adapter further includes an orifice disposed in its sidewall. This orifice may be covered by a latch door. This includes a door, latch and cotter pin which is used to seal tightly against the orifice in the housing wall of the sounding tube adapter. By opening and closing the latch door, the orifice may be exposed to allow measurement of liquid levels by use of a plumb line. When this door is opened, and the orifice exposed, plumb line measurement may be performed without the need of removing the level sensing apparatus from the sounding tube. This minimizes time and expense in measurement. When closed, the latch door seals tightly against the wall of the housing of the sounding tube adapter, thereby allowing for very accurate and reliable radar measurement. The use of the latch door also eliminates the need for a xe2x80x9cYxe2x80x9d-shaped fitting which would interfere with radar pulses, thereby reducing the accuracy of liquid level measurement.
Thus, in accordance with the present invention, when the latch door is in its closed position, the pulse radar of the level sensing gauge travels the sounding tube unimpeded, thereby resulting in a very accurate and reliable reading of the fluid level. The latch door and orifice in the sounding tube also prevent the need for removing the level sensing gauge from the sounding tube in order to measure fluid levels manually. This eliminates a cumbersome and time consuming process. Additionally the use of the apparatus of the present invention reduces the health hazards attendant the measuring process by minimizing the amount of noxious fumes which emit from the sounding tube.
By virtue of the foregoing, there is thus provided an apparatus and method for liquid level measurement in a sounding tube. The aforementioned and other advantages of the present invention shall be apparent from the accompanying drawings and description thereof.